The invention relates to a magnetic bearing control device and to the use of a three-phase converter for magnetic bearing control.
Electromagnets are used for actively regulated magnetic bearing arrangements. Premagnetization and rapid regulation of the force-forming current are required for position stabilization, for this purpose.
Essentially, two versions of magnetic bearings such as these are known:
On the one hand there are magnetic bearings with a premagnetization winding by means of which a basic magnetic field can be produced. In this case, a control field winding also exists, by means of which a desired force is applied by means of a regulation technique to the object to be borne.
Furthermore, magnetic bearings exist which have no basic field winding, that is to say no windings by means of which a premagnetization can be set. The regulation technique which is used in this case is based essentially on the so-called difference drive.
In known magnetic bearings with a separate basic field winding and control field winding, the premagnetization field is produced by a constant current, which is normally generated by a first power amplifier. Field gain or field attenuation of the premagnetization field is achieved by means of a current in the control field winding, which is normally generated by a second power amplifier.
If, for example, a number N of magnetic bearings are now used on a magnet spindle, then the premagnetization current for all N magnetic bearings can be applied by the first power amplifier, as a consequence of which a total of N+1 power amplifiers are required in this case.
As already mentioned, a difference drive is performed for commercially available magnetic bearings without a basic field winding, requiring two power amplifiers, for each magnetic bearing, and therefore a total of 2×N power amplifiers.
FIGS. 1 and 2 respectively show a difference drive for a magnetic bearing and a magnetic bearing with both a basic field winding and a control field winding.
In this case, FIG. 1 shows two power amplifiers 12, 14 which are used to provide the difference drive for the control field winding 10. For this purpose, a premagnetization current i0 is fed to both power amplifiers 12, 14 and, in the case of the first power amplifier 12, the control current ix is added to this and, in the case of the second power amplifier 14, the control current ix is subtracted from the premagnetization current i0.
The constant current i0 is therefore applied as a premagnetization current to both coils of the control field winding 10 for the difference drive as shown in FIG. 1. By way of example, this current is assumed to be 10 amperes, which flows uniformly in both coils, as a result of which the current through the first coil is equal to that through the second coil. Physically, the two coils are generally arranged vertically one above the other. If the intention is now to apply a force to an object which is located between the coils, for example a rotor, then the current in one of the coils must be increased, and that in the other must be decreased. This is done by means of the control current, which increases the current in one coil by addition to the premagnetization current, and decreases the current in the other coil by subtraction from the premagnetization current. For example, if the control current is 2 amperes, then the first coil carries a current of 12 amperes, while the second coil carries only 8 amperes. This considerably amplifies the magnetic field in the first coil in comparison to the magnetic field in the second coil, increasing the force acting, for example upwards. This allows desired forces on the object to be borne between the coils to be set by means of the control current. In control engineering terms, the magnetic bearing 1 shown in FIG. 1 is therefore based on deliberately increasing the magnetic field of one coil and, associated with this, decreasing the magnetic field of the other coil, of the control field winding 10.
FIG. 2 shows, schematically, a further known magnetic bearing 3, in the form of a magnetic bearing with both a basic field winding 16 and a control field winding 10.
In contrast to the embodiment shown in FIG. 1, one additional power amplifier is required in this case in order to generate the premagnetization current for the basic field winding 16. However, the number of power amplifiers required in comparison to the embodiment shown in FIG. 1 is decreased the greater the number of magnetic bearings that are provided for the application, since the premagnetization current of all the magnetic bearings which are used can normally be produced by a single power amplifier.
Therefore, until now, specific and therefore expensive power amplifiers and associated regulators have been required for operation of known magnetic bearings.
One alternative to power amplifiers such as these is disclosed in laid-open specification DE 10 2004 024 883 A1, in which magnetic bearings can also be operated using standard converters in a machine-tool converter system.
However, in this case, only two phases of the converter are used to drive the magnetic bearings. N+1 converters are therefore required to drive the magnetic bearing system when there are N magnetic bearings with premagnetization windings. If these magnetic bearings are not fitted with any premagnetization windings, 2×N converters are required.